Molecular Probes proposes to develop applications in basic and applied research and in medical diagnostics for the fluorescent latex microspheres that it has prepared. The superiority of these products in terms of their range of size, wavelength and fluorescence intensity will enhance their application in areas of research and industry that have sensitivity requirements that currently available fluorescent latex and other fluorescent reagents cannot meet. The new microspheres will be prepared from oil-soluble fluorescent dyes recently developed by Molecular Probes research. The specific applications the investigators propose to develop are: 1) new fluorescent latex-based hybridization reagents; 2) labeling reagents with improved fluorescence intensity, greater Stokes Shifts and multiple excitation/emission for better detectability and multi-color applications in imaging and flow cytometry; and 3) new, ultra-sensitive fluorescent probes for low-density cellular receptors. Most of the work envisioned in this proposal will be carried out by Molecular Probes research personnel, with assistance from outside collaborators.